This invention relates to an information processing system for transmitting information mutually between optional information processing stations by connecting in cascade via information transmission lines plural information processing stations dispersed over a wide area such as for example in a system for controlling objects that are dispersed over a wide area, such devices usually arranged in groups. Each group comprises an information processing station for mutually transmitting information between each device within the group, and the processing station of the group is connected by means of information transmission lines to the processing stations of other groups. In such a system, since the number and total cost of transmission lines would be great if each station is individually directly connected to every other station, it is conventional to connect plural information processing stations in cascade, either open loop or closed loop, as shown in FIGS. 1(a) and 1(b). The stations are shown as S.sub.1 -S.sub.n, the transmission lines are shown as L.sub.1 -L.sub.n.
A conventional arrangement for each processing station S.sub.1 -S.sub.n is shown in general block form in FIG. 2. A common information bus B is controlled by a controller Sc, and one or more information processing devices D.sub.1 -D.sub.n and one or two communication terminals Ta and Tb havng bilateral communication facility are connected in parallel to said bus. To each of the communication terminals Ta and Tb, external transmission lines La and Lb are connected via change-over switches Ra and Rb, respectively, for switching the lines to the by-pass transmission line L.sub.p when the station is out of order or is not operating. The other ends of lines La and Lb are connected to the communication terminals of other stations not shown in the figure.
In such stations, all information transmission, between the information processing devices and the communication terminals is carried out via bus B under control of station controller Sc. When received information contains the address of the receiving station and an information processing device within the station, the information is transmitted via bus B from the terminal where received to the designated device or devices, under control of the controller Sc. The controller reads the address of the received information and commands the intake of the information on the bus B to the information processing device to which it is addressed, or commands the intake of the information on the bus B to either terminal Ta or Tb for retransmission if the address is that of an information processing device in other than the station to which the controller belongs. When the communication terminal thus receives the information from the bus B, it feeds the information to an external transmitting line automatically and thereby transmits the information to another station. When the communication terminal receives information transmitted from an external transmitting line, it feeds the information to the bus B by permission of the controller Sc.
Therefore, when information is received from another station via line La, the communication terminal Ta feeds the information onto the bus B by permission of the controller Sc in the conventional manner. The controller Sc then reads the address of the information and if the address is that of the processing device D.sub.1 located at the station, for instance, it commands the processing device D.sub.1 to accept the information. If the address is that of a station other than the one to which the controller Sc belongs, it commands the other communication terminal Tb to accept the information and to transmit same via external transmitting line Lb to the next station along line Lb.
If the station stops operating for any reason, such as an electric failure, the change-over switches Ra and Rb operate and switch the contacts from the communication terminals Ta and Tb to the by-pass transmitting line L.sub.p to connect the external transmission line La and Lb with the by-pass transmission line L.sub.p and to disconnect the station S from the transmitting system.
In the information transmission process for systems shown in FIGS. 1(a) and 1(b), when information is dispatched from one station to another station, each station following the one station reads the address of the information and if the address is that of itself, it accepts the information in itself, and if the address is that of another station, it passes the information to the following stations. The information therefore cycles in the series or closed loop transmission line until the information reaches the addressed station. In systems of the type wherein plural stations are connected in cascade with open connection between both end stations S.sub.1 and S.sub.n, the end stations S.sub.1 and S.sub.n differ slightly from the other stations. That is, each of the stations S.sub.1 and S.sub.n has a communication terminal which is operated to receive the information transmitted from stations S.sub.2 and S.sub.n.sub.-1, respectively, to read the address of the information, and to return the information to stations S.sub.2 and S.sub.n.sub.-1, respectively, unless the address is that of itself. The function of reversing the direction of transmission of the information may be provided to each intermediate station in the processor of series open loop constitution shown in FIG. 1(a) or each station in the processing system of closed loop constitution shown in FIG. 1(b).
Since each station not in working condition passes the information through the by-pass transmission line, the performance of transmission between operating stations is not impaired because of a shut down of one or more stations in the loop. However, if the transmission of information is performed without knowing which stations are shut down it is possible that information addressed to a non-working station will be dispatched with the result that the information will continue to circulate through the loop. If such ineffective information is accumulated, the transmission efficiency of necessary information becomes worse, and the transmission of information between stations becomes delayed.
The object of this invention is to prevent the occurrence of ineffective information from being dispatched in a system of the type described above. One system and method for accumulating and disseminating common data showing the working condition of all stations is described and claimed in commonly assigned copending application (corresponds to Japanese Patent Application 50-17628) entitled "A Method and Apparatus for Transmitting Common Information in the Information Processing System".